In the course of manufacturing paper and similar products, including paper board and the like, it is well known to incorporate quantities of inorganic materials into the fibrous web in order to improve the quality of the resulting product. A number of inorganic materials have long been known to be effective for these purposes, such as titanium dioxide, which can be incorporated into the paper in the form of anatase or rutile. Titanium dioxide, however, is among the most expensive materials which are so usable. Accordingly, in recent years, considerable efforts have been made to develop satisfactory replacements for the said titanium dioxide.
Among the materials which have thus found increasing acceptance as paper fillers are substantially anhydrous kaolin clays. Materials of this type are generally prepared by partially or fully calcining a crude kaolin clay, which may have been initially subjected to prior beneficiation steps in order to remove certain impurities, e.g. for the purpose of improving brightness in the ultimate product. It is important for an understanding of the present invention, to recognize that those skilled in the art of kaolin processing draw a sharp and fundamental distinction between uncalcined and calcined kaolins. With respect to terminology, it is noted that the prior art literature, including numerous of the prior art patents relating to the field of kaolin products and processing, often uses the term "hydrous" to refer to a kaolin which has not been subjected to calcination--more specifically, which has not been heated to temperatures above about 450.degree. C., which temperatures serve to alter the basic crystal structure of kaolin. These so-called "hydrous" clays may have been produced from crude kaolins, which have been subjected to beneficiation, as, for example, to froth flotation, to magnetic separation, to mechanical delamination, grinding, or similar comminution, but not to the mentioned heating as would impair the crystal structure.
In an accurate technical sense, the description of these materials as "hydrous" is, however, incorrect. More specifically, there is no molecular water actually present in the kaolinite structure. Thus, although the composition can be (and often is) arbitrarily written in the form 2H.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2, it is now well-known that kaolinite is an aluminum hydroxide silicate of approximate composition Al.sub.2 (OH).sub.4 Si.sub.2 O.sub.5 (which equates to the hydrated formula just cited). Once the kaolin is subjected to calcination, which, for the purposes of this specification means being subjected to heating of 450.degree. C. or higher for a period which eliminates the hydroxyl groups, the crystalline structure of the kaolinite is destroyed. As used in this specification, the term "calcined kaolin" shall refer to such a kaolin. Preferably the calcined kaolin has been heated above the 980.degree. C. exotherm, and therefore is "fully calcined", as opposed to having been rendered merely a "metakaolin". Reference may be had in the foregoing connection to Proctor, U.S. Pat. Nos. 3,014,836 and to Fanselow et al, 3,586,823, which disclosures are representative of portions of the prior art pertinent to fully calcined kaolins.
A calcined product having characteristics generally superior to previously available such calcined kaolin pigments, is the ALPHATEX.RTM. product of E.C.C. America Inc., assignee of the present application. This product again is a substantially anhydrous white kaolin clay pigment, which has unusual efficacy as a filler in paper sheets and similar paper products. The pigment also has application as a coating pigment for paper, and as a pigment in paints and other filled systems. It generally consists of aggregates of anhydrous kaolin clay particles, and exhibits exceptionally high light-scatter and opacifying characteristics when incorporated as a filler in paper.
ALPHATEX.RTM. is further described in U.S. Pat. No. 4,381,948 to A. D. McConnell et al, as being an anhydrous white kaolin clay pigment having high light scattering when incorporated as a filler in paper, the pigment consisting of porous aggregates from sub-micron sized kaolin clay platelets obtained by classification of a dispersed kaolin clay to a 100% less than one micron ESD fraction, the aggregates having an average specific gravity in the range of 0.5 to 0.6 and a mean internal pore size of less than 0.55 microns. The size distribution of the aggregates is such that no more than 5% by weight thereof are greater than 10 microns ESD, at least 75% are of less than 2 microns ESD, and not more than 15% by weight are of less than 1 micron ESD. The pigment has a Valley abrasion value below 30 mg., and a G.E. brightness of at least 93.
Calcined kaolin clay products such as ALPHATEX.RTM. are normally pulverized and then air-classified after calcination for the purpose of removing +325 mesh residue (to conform to specification for intended use in paper), or in order to remove larger abrasive particles. Such products are then sold by the manufacturer as a finely pulverized low-bulk density powder, which powder, because of flow characteristics, is often difficult to handle by conventional bulk handling systems. Because of the difficulties in handling such products, they are typically shipped in bulk in "sparger cars". These are bulk-hopper railroad cars fitted with special valves at the bottom which allow water to be injected into the car upon arrival at the customer's facility. Air is then injected into the car to agitate the water and powdered clay mixture. As soon as possible after the injection of the water and air, the fluid suspension is pumped from the car at about 30%-35% solids into a storage tank.
Because of the low-bulk density of the clay powders, typically only 35 to 40 tons of clay can be loaded into a 4,700/ft.sup.3 rail car. The bulk density of this material would normally be measured in the laboratory to be about 10 to 12 lbs/ft.sup.3 ; this material would pack to about 15 to 17 lbs/ft.sup.3 in a fully loaded railroad car.
It has been disclosed in Bilimoria et al. U.S. Pat. No. 4,693,427 that the bulk density of calcined kaolin clay may be increased by dry milling the calcined and ground clay under specified conditions and in a specified way.
According to Cook et al, U.S. Pat. No. 4,561,597, bulk densities are increased by dry ball milling clay powder followed by pulverizing in a high-energy impact mill.
In procedures such as these, bulk densities have been significantly increased, but still remain below 25 lbs/ft.sup.3. Furthermore, the flowability characteristics of the dry powder still remain relatively poor.